Investigations were undertaken to further define those factors in cell-free and whole-cell systems which influence the rate and direction of protein synthesis. Emphasis is to be placed on the isolation of a biologically active messenger ribonucleoprotein from mammalian cells and on the purification and identification of the ribonucleic acid and protein components. The polypeptide product of cell-free and whole-cell culture systems stimulated by the ribonucleoprotein will be isolated and identified in an attempt to further characterize the factors which influence the rate and specificity of the polypeptide product.